"No compatible GPUs found" - Arch Linux
You might need to install the proprietary AMDGPU-PRO drivers, or at least the OpenCL part of it. The quick fix is to install opencl-amd from AUR:

git clone https://aur.archlinux.org/opencl-amd.git
cd opencl-amd
makepkg -s
sudo pacman -U opencl-amd-*-x86_64.pkg.tar.xz


(You might have to downgrade libdrm for opencl-amd to work, see here.)

Note that Arch Linux is not officially supported by AMDGPU-PRO.

Resources


Arch Wiki: "Blender does not show the AMD card as an OpenCL rendering device"
Arch Wiki: AMDGPU PRO